Conventionally, a SAW (Surface Acoustic Wave) device has been well known as one of the devices that use acoustic waves. The SAW device is used in a variety of circuits that process radio signals in a frequency band of 45 MHz to 2 GHz, which is commonly used in mobile phone terminals, for example. Such circuits are used in a transmission band-pass filter, a reception band-pass filter, a local filter, an antenna duplexer, an IF filter, an FM modulator, and the like.
The mobile phone terminals as described above are often used in various temperature environments, outdoors and indoors. Thus, the SAW device needs to have a feature capable of operating stably in various temperature environments. JP 2003-209458 A discloses an acoustic wave element in which an oxide silicon film having a temperature characteristics code different from that of a piezoelectric substrate is formed on the piezoelectric substrate. However, the configuration disclosed in Patent Document 1 has a problem in that the acoustic wave element becomes large.
JP 2004-112748 A, International Publication WO 98/052279, and a Non-Patent Document (Masatsune Yamaguchi, Takashi Yamashita, Ken-ya Hashimoto, Tatsuya Omori, “Highly Piezoelectric Boundary Waves in Si/SiO2/LiNbO3 Structure”, Proceeding of 1998 IEEE International Frequency Control Symposium, (US), IEEE, 1998, pp. 484-488) disclose an acoustic wave element using Love waves, and an acoustic boundary wave element using boundary waves that propagate between boundaries of different media. Any of these configurations can achieve an improvement in temperature characteristics and the downsizing of the acoustic wave element.
The acoustic wave elements and the acoustic boundary wave elements disclosed in JP 2003-209458 A, JP 2004-112748 A, International Publication WO 98/052279, and the Non-Patent Document are less reliable.